Another Motorola VK-106 restoration attempt

[Kevin Kuehn]

Originally Posted by pdr-fml

My other question is can I feed the coax rf output (channel 3 or 4) from this box into the RF converter/mixer grid and expect a good signal or is it too weak. This would be bypassing the RF amp tuner stage where I think I may have a problem.

You can not run the rf channel 3 or 4 directly though the mixer because it will not provide the needed IF frequency. The correct IF frequency is generated by the rf channel frequency beating with the receivers local oscillator freq. See below for more info.

Originally Posted by pdr-fml

I have successfully injected a 21.9MHz (audio IF for my set) FM modulated signal from my signal generator into the grid of the mixer and did alignment on sound IF (atleast I think I did - everything sounded good afterward). After I did the sound alignment my next check was going to be to see if I could get good audio using converter box. Well it did not work. All I got was buzzing noises. I am going to reverify my signal gen output accuracy but I think it is dead on it is a HP synthesized model. I even jacked around with the local osc adjustment for channel 3 and all I got was several FM radio stations coming thru nicely. I tried viewing signal with oscilloscope and never found anything that looked like video signal. So am I going to have to go thru tuner and RF amp?

You can set your generator to the video IF frequency and check the mixer and IF stages, similar to how you did the sound IF alignment. Just make sure you have the channel set to a blank position between 2 and 13, to disable the local oscillator. You may also use your signal generator as a video test bar generator by AM modulating the IF frequency with 1200hz. If 1200hz is not possible it's OK to use 400 or 1000hz. 1200hz in theory will give you 20 horizontal test bars on the CRT screen. The AM modulation beats with the receivers vertical sync oscillator and provides black and white test bars on the CRT screen. Normally you would start at the Video IF closest to the detector and work your way back towards the antenna input. You can even set the frequency generator to any channels video carrier(61.25mhz for ch 3) and connect it to the antenna input(same AM modulation will produce bar pattern). In this instance you do need to tune in the channel so that the local oscillator will beat with the video carrier freq and produce the correct IF.

It's always a good idea to use a coupling cap in series with you generator output just in case you accidentally hook into some high voltage source. Generally you'll be feeding the signal into the the grid of each stage.